Building a Sea Chest

[Caymanboy]

5200.

[mbmcdermott18]

Here is a picture of the sea chest assembled I will take some more detailed pictures tommorow as I install.

[Caymanboy]

I'm confused, (which would not be the first time!).
A sea chest to me, is a single inlet in the hull (bottom) with one valve, which then either has a box or pipe, which then in turn has seacocks and hoses going from it to the various compents you wish to supply, IE:, A/C intake, generators, main engines, washdown pumps, etc. That way in the event of a catastrophic failure, you close one valve. It also gives you the ability to have only one sea strainer.
The picture I see, is a sump tank.
What are your intentions for what is in the picture?

[Caymanboy]

Sorry, went back to the first post.
My concern would be the supplys you have, appears they are either 3/4" or 1"? That would be a total of 1 1/2" or 2", is that enough to supply ALL three of those pumps when they are running? They are what, 1500-2000 GPH apiece?
The next thing to figure out is, what kind of p/u's do you have? If there is no scoop on them, the water will get pulled out of the box, if there is, water will be pushed into the box, all the time.

[mbmcdermott18]

Cayman, I share your concern on the inlet size. I would prefer to have two 1.5" or 2" inlets. MY boat has two 1" raw water intakes that are glassed into the hull, changing the size of the intakes would be a much larger indertaking. I am not opposed to it but I would like to make sure it will not work like this.

As far as the design goes it should work as follows:

the two one inch raw water through hulls will be plumbed to a gemlux ball valve then hose ran aprox 18" from the valve to the 90 degree through hull on either side of the chest.

The pumps in the chest are plumbed to two livewells. One livewell has two pumps. The are connected to a thread to barb adapter which threads into the groco flanged adapters which have gemlux ball valves and are then plumbed to the livewell.

The chest will have a 1/8" mini ball valve in the lid to allow you to vent the chest.

the vlalve in the rear of the chest is going to the shurhold washdown pump.

[mbmcdermott18]

After a couple test runs this weekend I think I have this setup dialed in.

Put the boat in the water on Friday for the first test and learned two things:

a) the cable clams sealing the wires would leak a very small amount of air into the chest when the pumps were pulling a vacuum. Since the pumps can only pump water the air/water ratio slowly dropped bc of this air leak. No problem, undid the cable clams and filled the void between the two parts with 4200.

b) when underway the intakes are bringing a 50/50 air/water froth. Again since the pumps can only evacuate water we began to drain the chest. When you are running there is positive pressure in the chest so the simple fix was to vent the chest. this allowed the froth to enter the chest quicker then the air could be vented off.

decided to use a check valve to vent this chest automatically, we tested the next day with a check valve installed and it worked nicely.

As discussed earlier the size of the intakes had me concerned. This could have been a problem and was in the beginning. In talking with a few people it seems as though the majority of these setups are not air tight. I believe this brings the intake size into play.

Initially before I filled the cable clams with 4200 they were sucking air. Now anyone who has installed a cable clam know how effective they are at sealing. They would barely leak a drop of water. When you put the pumps on you are dropping the pressure inside the chest and it is going to draw something from the point of least resistance. Apparently the intakes were only the path of least resistance up to a point where it would start bringing in air from the wires and water from the intake. Once I sealed them up better the path of least resistance is now the intakes only and it just pulls water in faster. So really I think the key to the hole thing is being able to pull a good vacuum inside the chest. if there was no vacuum a 2" hole can flow just shy of 1600 gph at rest. these two 1" holes are flowing over 2400 gph at rest.

Now when you are on plane running 40/50/60 mph it is the opposite. the froth is coming into the intakes as fast as it can building a strong positive pressure. The key now is to get the air out as quick as possible in order to keep the chest full of water. When running the check valve opens up and air/water come out of the attached 3/8" hose at a pretty rapid pace. I believe I am going to mount a flush fuel vent somewhere on my euro transom and plumb the check valve to it so it sprays the water overboard.

Below is a picture of the chest just sitting in the bilge. I will permanently mount the chest this weekend hopefully. I will also be replacing all the livewell hoses.

[mbmcdermott18]

Finally fitted the last piece of the puzzle last week. I had been trying several different valves in conjunction with a check valve to let air out, keep water in, and hold a vacuum. Bottom line is the "air eliminators" that I found did not flow enough CFM to keep the chest full when on plane. Since the pumps can only move water air would slowly build in the chest until it was dry. Perhaps more than one would have done the job but I didn't want to drill more holes in my lid on a hunch.

Decided to eliminate the air eliminator and just plumb the vent overboard because it will vent a small amount of water. I had planned on using a gemlux flush vent to do this but I could not get my hands on one.

After a little thought I ran the hose in between the liner and cap and it is hidden behind the cap pointing down into the drain channel. The little bit of water that comes out of the vent hose should quickly escape through the scupper. Here are a few finished pictures. Thanks so much for the help. I have taken what I learned from this project and tackled a few other fabrication projects I had been wanting to complete.